module.exports = function (xs, fn) {
      var res = [];
      for (var i = 0; i < xs.length; i++) {
          var x = fn(xs[i], i);
          if (isArray(x)) res.push.apply(res, x);
          else res.push(x);
      }
      return res;
  };
  
  var isArray = Array.isArray || function (xs) {
      return Object.prototype.toString.call(xs) === '[object Array]';
  };